AB^2=(m^2+1)^2=m^4+2m^2+1
AC^2=(m^2-1)^2=m^4-2m^2+1
BC^2＝4m^2
∵BC^2+AC^2=AB^2
∴△ABC为RT三角形
∴∠C=90°,即∠ACB=90°
楼主是不是题目错了?∠ABC是无法求得的,只能求出∠ACB=90°